If the type error is at an implicit argument, we annotate
application with `pp.explicit := true`
Given the type incorrect definition
```
def f {a b c : α} : a = c :=
Eq.trans (a := a) (b := b = c)
```
We now generate the error
```
error: application type mismatch
@Eq.trans α a (b = c)
argument
b = c
has type
Prop
but is expected to have type
α
```
@Kha Note that we only enable `pp.explicit := true` for the relevant
application. That is, we set `pp.explicit := false` for each children.
Unfortunately, there is a corner case.
```
set_option pp.explicit true
def f {a b c : α} : a = c :=
Eq.trans (a := a) (b := b = c)
```
produces the error
```
error: application type mismatch
@Eq.trans α a (b = c)
argument
@Eq α b c
has type
Prop
but is expected to have type
α
```
The reset `pp.explicit := false` overwrote the user option.
I think the simplest solution is the following
1- The delaborator saves the initial set of Options `Init`
2- When it finds a node annotated with a `pp` options, it only
consider the option if it is not set by `Init`.
What do you think?
@Kha This one required a bunch of manual fixes. The main issue is that
before we added the string interpolation feature, we created
`MessageData`s using `++` and coercions. For example, given
`(e : Expr)`, we would write
```
let msg : MessageData := "type: " ++ e
```
and rely on the coercions `String -> MessageData` and
`Expr -> MessageData`, and the instance `Append MessageData`.
However, heterogeneous operators "block" the expected type propagation downwards.
This kind of code is obsolete now since we can write a more compact
version using string interpolation
```
let msg := m!"type: {e}"
```
@Kha I was tired of writing `arbitrary _` :)
There 0 places in the stdlib where the type needs to be provided.
If in the future we need to specify the type we can use
`arbitrary (α := <type>)`
